1. Field of the Invention
The present invention relates to a controller, which controls driving of an endless belt such as an intermediate transfer belt for use in an image forming device such as a copying machine or a printer, and an image forming device.
2. Description of the Related Art
An image forming device is known as an electrophotographic color image forming device, in which toner images of respective colors formed on a plurality of photoconductor drums, respectively, are sequentially transferred (primary transfer) on a rotatable endless intermediate transfer belt to be superimposed thereon, and the color image transferred on the intermediate transfer belt is transferred on a recording medium such as paper by a secondary transfer member.
In such an image forming device, if the running speed of the intermediate transfer belt varies, positions onto which the toner images of respective colors are superimposed, respectively, are misaligned, resulting in an image failure such as a color shift image. In order to prevent the variation in the running speed of the above intermediate transfer belt, a driving system for the intermediate transfer belt includes a belt driving controller (for example, reference to JP2000-356936A).
In the invention described in JP2000-356936A, the running speed of the image forming belt (intermediate transfer belt) is controlled according to the speed information filtered by a filtering unit which enhances a specific frequency component from the running speed information of the image forming belt detected by a detector.
In the invention described in JP2000-356936A, the influence of the variation in the thickness of the belt and the eccentricity of the driving roller relative to the running speed of the intermediate transfer belt (hereinafter referred to as a belt) can be controlled by feedback-controlling the surface speed of the belt. The influence of regular disturbance such as friction relative to the running speed of the belt can be also controlled by feedback-controlling the speed of the motor. However, in order to control the influence of the variation of the reduction mechanism and the rotation of the driving motor relative to the running speed of the belt, it is necessary to compensate the influence by means of the feedback control system for the belt surface speed. For this reason, the control loop of the feedback control system for the belt surface speed is required to respond to a frequency higher than the variation of the reduction mechanism and the rotation of the motor.
However, it is necessary for a response frequency of the control system to be higher than a mechanical resonance frequency (generally, 1000 Hz or below) for use in the image forming device, in order to avoid the resonance of the belt driving system for use in the image forming device. Because of this restriction, the response frequency of the control system required for controlling the influence of the variation of the reduction mechanism and the eccentricity of the motor can not be used in the control loop of the feedback control system for the belt surface speed. For example, if the resonance frequency of the belt driving system is 500 Hz and the rotation frequency of the motor is 40 Hz, the response frequency of the control system which can be set is 500 Hz or more. For this reason, the disturbance of 40 Hz by the motor can not be controlled by the control loop of the feedback control system for the belt surface speed.
Since the variation of the reduction mechanism and the eccentricity error of the motor can not be detected by an encoder which detects the rotation speed of the motor, the influence of the variation of the reduction mechanism and the eccentricity of the driving motor relative to the running speed of the belt can not be controlled even when a high response frequency can be set.
In JP2000-356936A, a filter for controlling the eccentricity of the driving roller is disposed in series in the feedback control system for the belt surface speed, but it is required to prepare a parameter of the filter for each speed mode in the image forming device including a plurality of speed modes. If the filter is disposed in the feedback control system which can not increase the response frequency, failures occur such as the increase in the influence of the phase lag or the interference with the mechanical resonance frequency of the belt driving system. Therefore, the factor which destabilizes the feedback control system is further increased.
FIGS. 25A, 25B are graphs each illustrating a frequency response of a belt surface speed (a) and a driving axis speed (b). As will be noted from the figures, when the frequency is the mechanical resonance frequency of the intermediate transfer belt or more, the phase of the belt surface speed delays, and the eccentricity of the reduction mechanism and the rotation of the motor relative to the running speed of the belt can not be controlled only by the feedback control for the belt surface speed.